This invention relates generally to sol-gel processes for producing dry gel monoliths that subsequently can be sintered into glass articles and, more particularly, relates to sol-gel processes of this kind using molds specially configured to enhance the process' effectiveness.
Substantial efforts have recently been expended in developing improved sol-gel processes for fabricating high-purity monolithic articles of glass. In such processes, a desired solution, i.e., a sol, containing glass-forming compounds, solvents, and catalysts, is poured into a mold and allowed to react. The solution typically includes tetraethyl orthosilicate, water, an alcohol, and an acid and/or base catalyst. Following hydrolysis and condensation reactions, the sol forms a porous matrix of solids, i.e., a gel. With aging, the gel shrinks in size by expelling fluids from the pores of the gel. The wet gel is then dried in a controlled environment, typically by removing the gel from the mold and placing it into an autoclave for subcritical or supercritical heating. The dried gel then is sintered into a solid monolith.
Advantages of the sol-gel process include chemical purity and homogeneity, flexibility in the selection of compositions, the ability to process at relatively low temperatures, and the producing of monolithic articles close to their final desired shapes, thereby minimizing finishing costs.
The efficiency of the process can be enhanced if the steps of gelling, aging and drying all are carried out within a single chamber and without the need to remove the gel from the mold. The need to remove the gel from the mold at an intermediate step of the process not only requires mechanical handling of the fragile gel and mold, but also lengthens the processing time. This is because removing the gel from the mold following the step of aging can be performed only after the gel has cooled to room temperature from its aging temperature, e.g., 60.degree. C.
An important factor bearing on the ability to perform the entire sol-gel process without removing the gel from the mold is the nature of the material from which the mold is formed. The ideal mold material should have good release characteristics, such that the fragile monolithic gel can be removed from the mold without damage.
The mold material also should be inert to attack from chemicals used in the sol-gel process, e.g., acid catalysts such as hydrochloric acid (HCl) and base catalysts such as ammonium hydroxide (NH.sub.4 OH). This requirement effectively precludes the use of molds formed of metal, because metal impurities could be leached from the mold and trapped in the gel, thus being retained in the glass monolith. Metal impurities retained within a glass monolith are particularly undesirable, because they can reduce the transmission of ultraviolet light. Such leaching also can reduce the mold's life span.
If the gel is to be dried while still located within the mold, the mold material must be able to withstand typical drying temperatures, e.g., 200.degree. C. and above. This means that the mold must not decompose at such temperatures and it should not deform when repeatedly cycled between room temperature and the maximum drying temperature. This requirement effectively excludes the use of molds formed of common polymeric materials such as polymethyl pentane and Teflon, which have softening temperatures substantially lower than 200.degree. C.
Some refractory ceramics, e.g., silicon carbide, boron nitride and carborundum, can survive the required high temperatures and are reasonably inert, making them suitable for use as mold materials. However, such materials are difficult and expensive to machine into the required mold shapes. In addition, these materials do not generally have good release characteristics, and gels can sometimes adhere to molds made of these materials.
It should therefore be appreciated that there is a need for a sol-gel process in which the steps of gelling, aging and drying all are carried out without removing the material from the mold. The present invention fulfills this need and provides further related advantages.